About 100 different purine derivatives and analogs were tested for their effect on protein synthesis and protein degradation in isolated rat hepatocytes. These included 6-aminopurines (adenine and adenosine analogs), 6-mercaptopurines, chloropurines, oxypurines, cytokinins, methylxanthines, methylindoles, benzimidazoles, and benzodiazepines. Most of the compounds were either inactive or inhibited protein synthesis as much as or more than they inhibited protein degradation. However, three methylated 6-aminopurines (3-methyladenine, 6-dimethylaminopurine riboside, and puromycin aminonucleoside) and four 6-mercaptopurines (6-methylmercaptopurine, 6-methylmercaptopurine riboside, 6-mercaptopurine riboside, and 2′,3′,5t - triacetyl-6-mercaptopurine riboside) had a markedly stronger effect on protein degradation than on synthesis, and might therefore be potentially useful as selective degradation inhibitors. None of the seven above-mentioned purines had any significant effect on the degradation of the exogenous protein, asialofetuin, and would therefore seem to selectively inhibit endogenous protein degradation. Since the degradation was not further affected by purines in the presence of amino acids or lysosomotropic amines, it is suggested that the purines exert their effect specifically upon the autophagic/lysosomal pathway. All the mercaptopurines significantly depressed cellular ATP levels, whereas the methylated aminopurines did not. For this reason, the latter are probably more useful as degradation inhibitors. 3-Methyladenine had no effect on protein synthesis at a concentration (5 m m ) which inhibited protein degradation by more than 60%, and may therefore be regarded as a highly specific inhibitor of autophagy.
